ABSTRACT Prion diseases are transmissible between animal-to-animal, animal-to-human and human-to- human, however, we still do not understand completely the mechanisms, factors and biological processes controlling the inter-individual transmission of this unique infectious agent. Among animal diseases, chronic wasting disease (CWD) represents a serious problem, because it continues to propagate uncontrollably among wild and captive cervids in North America. The risk of CWD transmission to humans is unknown which is a major concern because the number of sick animals and their geographical distribution is rapidly increasing. The mechanism by which CWD propagates so efficiently among cervids is also unknown, but recent findings have implicated environmental contamination with prions. The main goal of this project is to study the mechanisms controlling the species barrier, particularly to investigate the possibility that CWD may, under determined conditions, infect humans. We will also study the role of environmental contamination in CWD transmission, focusing on plants and various surfaces as vectors for prion propagation. In Specific Aim 1 we will investigate the role of prion strain adaptation in the zoonotic potential of CWD. The hypothesis for these studies is that the species barrier is a dynamic process that changes over time when prion strains mature and evolve. We will study the molecular mechanism of prion strain maturation, the biochemical and structural properties that differentiate CWD strains able and unable to convert human PrPC into PrPSc and analyze a large collection of natural CWD specimens to investigate differences on the strength of the human species barrier in vitro. In Specific Aim 2 we will test the hypothesis that the strength of the species barrier is lower in hosts harboring brain damage associated to other neurological diseases or even to subclinical or pre-clinical conditions. We will investigate whether the cervid/human species barrier can be altered in transgenic mice expressing human PrP by the co-existence of another brain abnormality, using established models of chronic neurodegenerative diseases (Alzheimer's and Parkinson's disease) and acute brain damage (traumatic brain injury and stroke). In Specific Aim 3 we will study the role of plants as fomites for prion transmission by analyzing prion binding to plants, retention of infectivity and transport of PrPSc from soil to different parts of the plants. Studies will be done using natural CWD prions and infectivity experiments will be performed in cultured cells, gene-targeted cervidtransgenic mice and the natural host (white tail deer). We will also test plants collected from CWD affected areas for the presence of PrPSc by PMCA. In Specific Aim 4 we will test the hypothesis that prions buildup in the environment by progressively binding to diverse environmental surfaces, where they can infect animals by simple contact. We will study the binding affinity of PrPSc to various surfaces (including stones, wood, plastic, glass, concrete, stainless steel and aluminum), the time permanence of PrPSc and infectivity in the surface and the effect of environmental changes on infectivity. We will also investigate the transmission of the disease through casual contact with contaminated surfaces and the mechanism by which this happen. The findings generated in this project will contribute to understand the zoonotic potential of CWD, the factors modulating the transmission of this disease, and the role the environment plays on prion transmission. These studies are essential to design measures to prevent further propagation of CWD, and to avoid the emergence of new diseases with potentially disastrous consequences.